1. Field of the Invention
This invention relates generally to an aperture correction signal processing circuit for processing an aperture correction signal used for improving the distinction of a reproduced picture of, for example, a colour television receiver, and more particularly to an improved aperture correction signal processing circuit by which an aperture correction signal free from noise can be obtained with simple construction.
2. Description of the Prior Art
In a television receiver, particularly in a color televsision receiver, aperture correction has been carried out in order to display a reproduced picture with distinct contour on the screen of the picture tube. To this end, an aperture correction circuit is provided on the video signal transmission path to produce an aperture correction signal. This aperture correction signal is added to the video signal to obtain an aperture-corrected video signal which is fed to the picture tube to display a clear picture on its screen. FIG. 1 shows the aperture correction circuit as mentioned above.
In the circuit of FIG. 1, reference numeral 1 designates an input terminal 1 to which the video signal is supplied. The input terminal is connected to the input side of a delay circuit 2 the output side of which is in turn connected to the input side of another delay circuit 3. The delay circuits 2 and 3 are designed to have the same characteristics or to have the same delay time .tau.. The output side of the delay circuit 3 is connected to one input side of an adding circuit 4, while the input terminal 1 is connected directly to the other input side of the adding circuit 4. The output side of the adding circuit 4 is then connected to one input side of a subtracting circuit 5, while the output side of the delay circuit 2 is connected to the other input side of the subtracting circuit 5. Further, the output side of the subtracting circuit 5 is connected to one input side of an adding circuit 6, while the output side of the delay circuit 2 is connected to the other input side of the adding circuit 6. Then, the output side of the adding circuit 6 is connected to an output terminal 7.
The video signal fed to the input terminal 1 is practically of a dull rectangular waveform due to characteristics of the transmission path and the like. In this case, however, the video signal is assumed to be a pure rectangular wave signal 8 as shown in FIG. 2A for the sake of easy explanation. This video signal 8 is supplied to the delay circuit 2 to derive therefrom a video signal 9 which is delayed by time .tau. as shown in FIG. 2B. The signal 9 is then applied to the delay circuit 3 where it is further delayed by another time .tau. to provide a signal 10, which is delayed from the video signal 8 by 2.tau. in total, as shown in FIG. 2C. Thus, the adding circuit 4 is fed with the signal 10 from the delay circuit 3 and the video signal 8 from the input terminal 1 thereby producing a signal 11 as shown in FIG. 2D. This signal 11 is next applied to the subtracting circuit 5 for being subtracted from the output signal 9 of the delay circuit 2 so that an aperture correction signal 12 as shown in FIG. 2E is obtained at the output side of the subtracting circuit 5. This aperture correction signal 12 is applied to the adding circuit 6 where it is added to the video signal 9 fed from the delay circuit 2 to provide at the output terminal 7 an aperture-corrected video signal 13 with its largely changing portions being preshot and overshot as shown in FIG. 2F. When this video signal 13 is applied to the picture tube in the color television receiver, a picture with distinct contour must be reproduced.
However, the aperture correction signal produced in the circuit of FIG. 1 is normally superposed with base noise as shown in FIG. 3 due to noise caused by band pass characteristics of the circuit, noise originally contained in the input signal, and so on. For this reason, the video signal corrected by the aperture correction signal 12 will be superposed thereon with noise. As a result, when this video signal is applied to the picture tube, the reproduced picture on the screen is adversely influenced by the noise and hence a proper aperture correction will not be achieved.